Apparatus and methods for determining spectral wavelength characteristics of radiation are known in the art, but they generally require relatively complex and expensive optical systems, detectors, and electronics. The complexity of the prior art systems either diminishes reliability and results in greater measurement-to-measurement and system-to-system variability or require expensive equipment.
Reliance on conventional filters, prisms, gratings, optical modulators, and so forth, to filter or separate wavelengths from a beam, has constrained the design of conventional systems. Inexpensive and compact configurations that are stable, have multiple measurement channels, and have the required dynamic range and sensitivity, are not available.
While dichroic or interference filters and beamsplitters have been used as components of the spectral filtration scheme in some of these conventional systems, they have had three significant attributes that limited the reduction in system cost, increased the size, and limited overall system performance. First, while these filters are used to selectively transmit certain wavelengths of radiation and reflect other wavelengths, they were not used with the intention to transmit and reflect fractional amounts of the same wavelengths. Second, the transition region between the spectral regions of high transmission and high reflection is spectrally narrow, usually about 60 nm or less, and the transfer characteristic of the transition region is usually non-linear, and of only incidental importance to performance. Third, the transfer characteristic in the wavelength regions on either side of the transition region is of primary importance and is optimized for the transmission or reflection of all the wavelengths in the respective regions.
A typical prior-art dichroic beamsplitter characteristic and its complement with these attributes is illustrated in FIG. 1, which shows a reflective region 22, a transition region 24, and a transmissive region 26. The sharp transition between .lambda..sub.2 and .lambda..sub.3 is important for achieving the desired color separation in the dichroic beamsplitter. The flat transfer characteristic between .lambda..sub.1 and .lambda..sub.2 and between .lambda..sub.3 and .lambda..sub.4 assure that the beamsplitter separates the beam but does not introduce additional intensity variation as a function of wavelength.
There is a need in numerous fields for a simple, compact, inexpensive, mobile, reliable, multi-channel system and method for determining the wavelength of a beam of radiation with high sensitivity. For example, in the field of gel electrophoresis, a multi-component fluorescent marker system is used to differentially and uniquely label sample constituents along one or more gel electrophoresis channels. The presence and identity of sample constituents at locations along each channel are determined from the emission wavelength of the associated marker at each location. The magnitude of the emission may vary over a large range but the intensity is low because the amount of marker must be limited based on the small amount of sample present. Therefore, an apparatus would have significant application that has a dynamic range of about four orders of magnitude, and sufficient sensitivity to detect the emissions from smaller than ten nanogram amounts of fluorescer, and further capable of accurately determining the wavelengths and, thus, the sample constituents corresponding to those emissions. A comparable need exists in the field of enzyme analysis. In the field of scanning systems, there is a need for a system that can rapidly determine the color of regions of a document using a single scan of the document, a minimum number of detectors, and a fast simple color determination algorithm.